In screw compressors, unloading is typically achieved by means of a slide valve which is reciprocated along an axis which is parallel to the axes of the helical rotors. The position of the slide valve relative to the rotors adjusts the size and duration of opening of the suction port by changing the effective length of the rotors and thereby the trapped volume. Because the slide valve position is thus directly related to the compressor output, the position of the slide valve has been sensed and a feedback signal provided to the control system which controls compressor capacity by positioning the slide valve. Additionally, an indicator can be positioned by the slide valve as it is moved.
In the past, the feedback and/or indicator structure ordinarily required the passage of a rod through the compressor casing. The rod would move with the slide valve and, although the rod is sealed, there have been problems due to leakage. The sealing problems were overcome in U.S. Pat. No. 3,738,116 which has the rod located in a sealed tube. The rod is in an inner tube and carries a magnetic member which coacts with an annular magnetic member surrounding the inner tube. The position of the annular magnetic member gives a visual indication of the slide valve position. Additionally, the annular magnetic member coacts with reed switches located in the inner wall of the outer tube when the annular magnetic member and therefore the slide valve is at one or more specific locations. This, however, does not satisfy the need for a direct, continuous output indicative of the slide valve position.